Techniques of the type mentioned above are previously known. In such known techniques the measuring of the moistness, i.e. the specific fluid content of the bed, is a big problem. Measuring based on measuring the conductivity by means of electrodes introduced into the bed is sensitive to disturbances, unstable and thereby extraordinary difficult to calibrate, amongst others because of oxidation of the measuring electrodes. Measuring based on capacitive measurements is not suitable for water-saturated substances.